MATLAB will do all of the loops in the order it encounters them. So for the k=1 iteration it will do the i=1:nx loop in its entirety and then do the i=nx:1 loop in its entirety. Then it will do the k=2 iteration and do both inner loops in their entirety again. For every iteration of k, both inner loops will be done in their entirety.
Side Note: If nx>1, that i=nx:1 loop won't do anything. Maybe i=nx:-1:1 was meant?